Reflective research

"Think globally, study locally" could be a mantra for Veerabhadran Ramanathan. The climate expert at Scripps Institution of Oceanography has been intrigued for years by how Earth maintains its balance between incoming sunlight and outgoing long-wave radiation. Now he's bringing the problem to cloud scale, with the help of small unpiloted aircraft that will hug the tops and bottoms of marine cumulus and stratocumulus and fly inside them.

The idea behind the Global Albedo Project (GAP) is to gather the first long-term, up-close measurements of how radiation gets reflected and absorbed by cloud-borne mixtures of pollution, water droplets, and natural aerosols such as dust and salt.

To carry out the precision flying needed for GAP, Ramanathan and senior advisor Joachim Kuettner (UCAR) are teaming with colleagues in aerospace engineering and instrument design. With an NSF grant, they're custom-building a set of lightweight robotic aircraft, also known as unmanned airborne vehicles (UAVs), and a set of miniaturized instruments weighing no more than 5 kilograms (11 pounds). Boasting a range on the order of 5000 kilometers (2700 nautical miles), the planes could lead to a new community fleet of research platforms. The project's findings are expected to help global modelers improve their depictions of clouds and aerosols.

How constant is Earth's reflectivity?

If there's a single number that gnaws at Ramanathan, it's 0.30. That's the best estimate of Earth's albedo, the fraction of incoming sunlight that gets reflected to space by clouds and the planet's surface.

The albedo of Earth has obvious and major implications for climate. If Earth reflected just 10% more sunlight (an albedo of 0.33), it could plunge the planet into a climate similar to that of the last ice age. If the albedo dropped to 0.27, the effect would be comparable to a fivefold increase in the atmospheric concentration of carbon dioxide, according to Ramanathan and Kuettner.

Less dramatic variations have emerged in observations and models. The global albedo apparently increased by as much as 2% (about 0.007) for two years after the 1991 eruption of Mount Pinatubo. And the second and third versions of the Community Climate System Model reduce albedo by about 3% (or 0.01) over the next 100 years, assuming a 1% annual rise in carbon dioxide concentrations. These data appear in an upcoming paper in the Journal of Climate by NCAR's Jeffrey Kiehl and colleagues.

Global climate models differ in their handling of albedo, given the diversity in how they treat clouds. A May 6 review in Science by Robert Charlson (University of Washington) and colleagues shows differences of more than 5% among the albedos produced by six major models in preindustrial simulations. The values range anywhere from 0.28 to nearly 0.32, depending on the model and the time of year. That's on par with the uncertainty in current estimates of global average albedo provided by satellite measurements.

What concerns Ramanathan and Kuettner is that nobody knows what short- or long-term variations the future may bring, as cloud patterns evolve and ice and vegetation adapt to a changing climate. "There could be a strong variation of the albedo that we don't anticipate," says Kuettner.

Adds Ramanathan, "We have practically no theory of why the planet's albedo should be 0.30. Given this state of the field, and given the fact that clouds exert a large global cooling effect, we need a new approach to cut through the current impasse on this fundamental problem in climate dynamics."

Also in the mix is a flurry of recent studies on "global dimming." Limited data from ground- and satellite-based radiation sensors show that less sunlight was reaching Earth from the 1950s to the 1980s—a drop of several percent per decade across land areas. The culprits seem to be pollution, mainly sulfates and other aerosols, and possibly changes in cloudiness, be it due to natural or anthropogenic causes. Aerosols can shield the sun either directly or indirectly; the latter occurs when they affect the brightness and extent of cloud cover.

"This indirect effect is acknowledged to be the largest source of uncertainty in understanding the human impact on global climate," says Ramanathan. The patchwork nature of dimming, strongest over industrialized parts of the Northern Hemisphere, also points to pollution.

Many scientists quietly acknowledged global dimming early on, but the phenomenon got little notice until the last few years, when better and longer-term data sets became available. New work published this spring in Science by Martin Wild (Swiss Federal Institute of Technology) and others shows a shift from dimming to brightening since the 1990s, perhaps due to cleaner industry. However, the trend hasn't yet erased all of the late 20th century's dimming.

Ramanathan's own field work supports the dimming hypothesis. "What we found in CEPEX [the 1992 Central Equatorial Pacific Ocean Experiment] was the amount of sunlight that hit the ocean surface was a lot less than the models were predicting. The atmosphere was a lot darker than we thought it would be." The Indian Ocean Experiment (INDOEX) in 1999 produced even more dramatic findings: winter smog blocked up to 20% of the sunlight over parts of the Indian subcontinent and nearby oceans.

For many years, says Ramanathan, researchers who dealt with dimming "were viewed with skepticism, if not contemptuously. The INDOEX results came, and they were delirious."

This schematic depicts the three-UAV configuration that will straddle low-level marine clouds during GAP (Illustration courtesy Scripps Institution of Oceanography.)

Up close to clouds

The global-dimming saga further piqued Ramanathan's interest in getting a better handle on Earth's albedo. "We collected a phenomenal amount of data in CEPEX and INDOEX, but they were expensive projects that lasted just a few weeks," Ramanathan says. "Clouds are such turbulent phenomena—we need to observe them all the time throughout the year. That's when the idea started germinating: there must be a better, cheaper, faster way. "

The concept that emerged for GAP was to send three instrumented UAVs in a column sandwiching a low-lying cloud. Staying close to the cloud surface is critical, says Ramanathan, because clouds can reflect sunlight in all directions, whereas a satellite detects only the sunlight reflected at the angle directly toward it.

The plan involves three aircraft stacked in a column less than 2 km deep. While flying at highway speeds, they'd stay within 2 km of each other horizontally for six to eight hours. Only UAVs can pull this off, says Ramanathan, especially given the long-term sampling ambitions of the study. "Undertaking this with manned aircraft would be prohibitively expensive and likely dangerous, since the flights could range for more than 24 hours," says Ramanathan.

Judith Curry. (Photo by Peter Webster.)

However, not just any UAV would do. The most widely used small UAV, made by Aerosonde, lacks the range and payload needed. Larger UAVs, many originating from the military, are too expensive. So Ramanathan and Kuettner decided to work with aerospace experts at the University of California, San Diego, to build their own UAV, with the side benefit of engaging students in the process.

UAVs are fast becoming a favored tool in undergraduate and graduate teaching, says Judith Curry (Georgia Institute of Technology). A frequent user of UAVs for her own research in the Arctic, Curry and her students are putting together a public Web site to serve as an information resource for NASA and university-based UAV efforts (see "On the Web").

At Georgia Tech, aerospace and atmospheric science students came together this summer to build a low-flying UAV that measured carbon monoxide and dust from the fields and highways near Atlanta. "Some good science can be accomplished with relatively small and simple aircraft," says Curry. She notes that larger UAVs aren't feasible for student work because of liability concerns.

Putting a university-built UAV into a major field project is something new, says Curry. "Ramanathan's project is far more ambitious than anything attempted so far by a university group in terms of building a UAV for atmospheric science applications," Curry says.

Georgia Tech aerospace engineering undergraduates (left to right) Sapan Shah, Matt Daskilewicz, and Tamil Periasamy work on the fuselage for the UAV they designed and built this summer for air quality measurements. (Photo by Adam Broughton.)

Even commercially built UAVs are only slowly making their way into large-scale research efforts. Since 2003 NASA has maintained a community fleet of UAVs at Wallops Island, Virginia, including a number of Aerosonde aircraft. However, Curry notes, "UAVs are still not really cost-effective relative to manned vehicles for the most part. The big advantage is if you're doing something we call ‘dull, dirty, or dangerous.' You wouldn't want to send a manned aircraft into a volcanic plume or really low in the boundary layer.

Building a better UAV

It's been two years in the making, but only now does the UAV prototype for GAP resemble an actual aircraft. A team of students under the direction of John Kosmatka, a UCSD professor of structural engineering, added wings to the fuselage in late June. "It's been a very exciting spring," says Kosmatka, who is a co-principal investigator in GAP along with Ramanathan.

The new plane is entirely built of carbon composites for lightness and strength. With the prototype now built, the die is literally cast for follow-ups. "It's all being done with permanent tooling, so it'll be easy to fabricate many more aircraft. It's the first one that takes all the time," says Kosmatka. Flight tests began in July, with longer-range missions expected later this summer.

At the same time, a group in Ramanathan's lab headed by Greg Roberts has been refining and integrating the miniaturized sensors for each UAV, including cloud nuclei counters, droplet probes, radiometers, and a video camera. "If you want to measure the cloud below, inside, and on top, you need pretty sophisticated equipment," says Kuettner. "This group has miniaturized the instrumentation in a really fantastic way."

The UAV's autopilot, a commercially available unit being customized by Optimum Solutions, makes the plane completely autonomous—an AUAV. This means that its complete mission can be preprogrammed using Global Positioning System navigation coordinates. Research data will travel to Earth via a modem and a telephone linked to the Iridium satellite network.

If instruments and aircraft are both ready, an initial test flight may head for Hawaii this fall before wintertime's strong westerlies kick in across the North Pacific. Then the GAP leaders turn their attention to the Maldives, where next February and March they hope to test the stacked-aircraft navigation strategy, using a different set of UAVs than the ones being developed at UCSD.

The first full-scale mission involving three of the custom-built UAVs should take place by the end of 2006, assuming all goes well. At that point, the plan is for a round-trip flight from San Diego to Honolulu and back about every 10 days for a full year. The flights will be coordinated with NASA's A-Train satellite formation, including Aqua, now in flight, as well as the soon-to-be-launched CloudSAT and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation).

Layers of uncertainty

Fellow researchers are watching GAP unfold with interest. "The UAV is an attractive size," says Curry. "It's fairly small and relatively economical but carries a decent payload. It fills a niche that seems to be missing in commercial UAVs." Despite this appeal, Curry also notes the challenge facing the aircraft: "There are a lot of UAV startups that never made it. I'll be very impressed if they get this to work."

Beate Liepert (Lamont-Doherty Earth Observatory), one of the lead researchers on global dimming, hopes that GAP will fill in blanks on the vertical distribution of pollution. As part of research soon to be published, Liepert and her students took to the skies near New York City. They brought photometers and particle counters aboard hot air balloons (the same type used for commercial balloon rides) and sailed through airborne grit and grime. Last summer they found several distinct layers of pollution, including a ground-hugging smog produced locally and a higher layer of smoke imported from Alaskan wildfires.

Left: Beate Liepert (foreground) oversees the readying of a balloon to gather data on air pollution near New York City in July 2004. To her far left is Peggy Hannon, a summer intern at Lamont-Doherty Earth Observatory. Above: Liepert (left) and LDEO interns Peggy Hannon and Colby Blitz monitor data-gathering devices and track the location of their balloon from an altitude of 4000 feet. (Photos by Carmen Alex, LDEO.)

"You can have the same amount of pollution, but it might have completely different effects on dimming depending on how it's layered," says Liepert. "Satellites and surface measurements only give you an integrated measure. It's so important to know where the aerosols are in the atmosphere in relation to the clouds. That's very important information you could get from this kind of study. "

Once GAP is up and running, Ramanathan and Kosmatka hope to establish a three-course sequence at UCSD that would cover UAV design and related atmospheric research and involve students in building a UAV. Ultimately, Ramanathan envisions a fleet of UAVs constantly plying the skies, not unlike the drifters and buoys that routinely track ocean currents.

"My vision is that we'll have hundreds of these flying over vast areas of the ocean," he says. "I'm hoping it will pioneer a new way of observing clouds and finally solve the problem of the statistical robustness of cloud physics data."

A busy eighth decade of research

GAP serves as the next chapter in Joachim Kuettner’s extraordinary blending of aviation know-how with atmospheric research. In the 1930s, Kuettner, now 95, employed 22 gliders to sample mountain waves for his dissertation in meteorology at the University of Hamburg. He’s since overseen numerous aircraft-based field projects, as well as the involvement of NASA’s Marshall Space Flight Center in the Mercury and Apollo projects.

Next spring Kuettner will assist coordinator Vanda Grubišic (Desert Research Institute) with the Terrain-induced Rotor Experiment. It’s the first major campaign involving HIAPER, the High-Performance Instrumented Airborne Platform for Environmental Research. It will also bring Kuettner back to the Sierra Nevada mountains, where he managed a U.S. Air Force field campaign in the 1950s and flew a glider to a record height of 43,000 feet.

In a 1999 UCAR Quarterly profile (see “On the Web”), Kuettner didn’t see any magic ingredient to his longevity as a working scientist: “I’m surprised myself. I think it’s primarily luck. Of course, you have to monitor your health as you monitor the weather. Above all, you have to keep your curiosity alive.”